Technical Field
This invention relates to liquid pumps, and in particular, the sealing of a liquid pump to prevent the liquid that is being pumped from coming in contact with the electric motor or other drive of the pump.
Description of Related Art
A pump typically is comprised of an electrical motor or other shaft driving mechanism mounted above a volute casing. The rotor shaft of the motor is connected to an impeller located in the volute casing. The electric motor rotates the impeller, which moves the liquid to be pumped. A housing surrounds the electrical motor, protecting it from moisture.
A major contributor to the cost of a pump is the electrical motor that drives the impeller. Protecting the electrical motor from moisture is very important and difficult to perform, because the motor shaft must extend beyond the motor enclosure and into the volute chamber, where it is connected to the impeller. During pumping of a liquid, the volute chamber is typically filled with pressurized liquid. Thus one or more seals must be provided on the motor shaft, which prevent the liquid being pumped from leaking along the shaft and into the motor housing where it would wet and damage the motor.
Historically, pump shafts have been sealed by the use of packing consisting of string, which has been soaked in tallow or similar grease, with a gland nut used to compress the packing into a stuffing box. Over the years “packing” has generally been replaced with mechanical face seals or lip seals. These newer devices have improved the seal ability but they are not without problems. A mechanical shaft seal can fail for a number of reasons. Solid debris such as sand can erode the sealing faces. Heat is another major source of seal failure. The sealing faces of a mechanical seal require lubrication to minimize the effects of friction. Installation of the seal and the alignment of the relative parts of the assembly also affect the life of a seal.
A seal failure is very costly. In the case of a single seal pump, only the volute wall separates the motor from the liquid being pumped, and thus the rate of heat transfer from the motor into the liquid is high. However, the risk of a seal failure and damage to the motor is also high, because failure of the seal results in direct contact of the motor with the liquid being pumped. Thus liquid entry into the motor enclosure can damage ball bearings, short the windings of the motor, and/or deteriorate the insulation of the motor. In some applications a seal failure could allow explosive liquid or gases to enter the motor enclosure and cause an explosion within the motor chamber. In these applications, an explosion proof pump is required.
For such applications in particular, pump manufactures offer dual seal pumps with a leak detection device located between the two seals. The benefit of this design is that the pump can be removed from service once the lower seal has been compromised. The liquid leak past the lower seal is detected before the liquid can leak past the upper seal and damage the motor. Normally the repair is simple, requiring only the replacement of the lower seal.
In providing dual seal pumps, pump manufacturers typically provide an enclosed cavity between the upper seal and the lower seal. This is done by adding an additional casting equal in diameter to the motor housing and located between the motor housing and the pump volute. The casting forms the enclosed cavity and also includes support for the upper and lower seals. The casting is also provided with a port and removable plug, so that the cavity can be filled with oil, so as to provide lubrication of the seals, and a better medium for detection of any leakage of liquid being pumped into the cavity.
However, the addition of the second seal and the oil-filled cavity has caused some difficulties with regard to cooling the electric motor of the pump. The electric motor that is used to drive the impeller of a pump creates a large amount of heat, which must be dissipated to the surrounding environment, which is either air, or for a submersible pump, water or another liquid. Some of the heat is dissipated out through the side wall of the motor casing. However, it is also highly beneficial to have a large amount of heat dissipated out through the bottom end of the motor through the volute wall, and into the liquid being pumped.
In current dual seal pumps having upper and lower seals, and an oil-filled cavity formed in a casting between the seals, the oil filled cavity is typically an annular cavity that occupies the entire volume between the motor shaft and the outer wall of the cavity and seal casting, and extends a full 360 degrees around the motor shaft. Thus the oil in the cavity acts as an insulating medium that reduces the rate of heat transfer from the pump motor axially through the upper pump volute wall and into the liquid being pumped through the pump volute. This reduced rate of heat transfer from the pump motor causes a problem in that due to sustained operation at higher temperatures, the life of the motor is reduced.
Thus there remains a need for a pump that has dual seal capability that reduces the risk of motor damage due to a seal failure, while also having a high rate of heat transfer out of the pump motor enclosure that reduces the risk of early motor failure from operating at a high temperature.